The invention concerns a process for manufacturing ribbed tubes, wherein a stainless steel tube is provided with ribs by helicoidally winding a copper strip around it, and ribbed tubes thus manufactured.
European Patent 0 303 074 and U.S. Pat. Nos. 4,841,617 and 4,969,255 disclose a process for manufacturing metallic heat exchanger tubes having a heat exchange surface area increased by ribs. In that process, a strip forming the ribs is helicoidally wound around the outside surface of the rotating tube with the bottom edge of the strip being continuously welded to the tube.
In that process, welding is done by melting the contact surfaces of the tube and the bottom edge of the strip, the tube being melted with a laser beam exclusively on the tube surface and the strip being melted with the same laser beam exclusively in the area where the strip comes into contact with the tube, and by then joining the bottom edge of the strip being fed to the tube.
The tube and strip surface areas being melted and joined are brought tightly together by the attraction force exerted on the strip being fed as the strip comes into contact with the tube and are bonded together when they are subsequently cooled.
This process has been proven effective in industry. In particular, when different grades of stainless steel are used for the tube and ribs, this known process is well-suited and is often used for manufacturing ribbed tubes for a great variety of applications.
However, when a stainless steel tube and copper ribs are used for manufacturing ribbed tubes, this process has proven to be difficult to perform and can only be applied using complex equipment.
The main cause of the problem resides in the different material properties of stainless steel and copper, in particular the different melting points with a temperature difference of approximately 400.degree. C. In this case there is a danger that the copper may sublimate before the stainless steel is melted.
This fact requires that the laser beam setting and control, as well as the rotation and winding speeds, be accurately adjusted. Corrective measures must also often be applied during the process for fine-tuning. Therefore the process, its control, and automation are complex.
Furthermore, the energy consumption of the laser, which must be focused on the surface of the tube and the bottom edge of the strip, is very high. Overall, the known process borders on the unfeasible when the materials stainless steel and copper are combined.